1. Field of the Invention
The invention relates to a method and a device for guiding steel bands about the longitudinal axis of a continuously working press for the production of particle boards, fiber boards, and plywood boards. More particularly, the claimed invention relates to such a press in which first and second bands are guided, via driving drums and return drums, around a pressing ram and a pressing table, respectively, and are supported on a plurality of co-rotating rods with an adjusting pressing gap therebetween.
2. Discussion of the Related Art
In heated-plate presses of this type, the pressing ram and the press table consist merely of web plates and transverse ribs connecting these web plates (see, for example, DE-A1-3,149,243, and U.S. Pat. No. 4,909,719 the disclosures of which are incorporated herein by reference). In this type of press, a plurality of web plates having transverse ribs are welded together to form individual spars which, in turn, by virtue of their arrangement next to one another and their attachment of heated plates, constitute the length of the pressing ram and the pressing table and thus of the heated-plate press. The lugs or projections protruding from the web plates on the left and on the right act as abutments for the tension frame press uprights in order to lift and lower the pressing ram. In addition, short-stroke pressing cylinder/piston arrangements are arranged between the upper crossheads and the lugs of the pressing ram.
The practical use of continuous presses, whether hydrostatic or rolling supports having rolling rods are employed, has shown that with the provision of increasingly long presses in order to achieve greater outputs, it is no longer possible to ensure the necessary steel-band control. The limit is a press length of about 28 m. With increasing lengths up to 40 m and more slight disruptions occur, for example:
a) disruptions caused by lubricating the steel bands and rolling rods; and
b) disruptions arising in the event of uneven bulk density distribution.
These disruptions have a disruptive effect on an on-line control of the press. Consequently, it is not always possible to guide the steel bands back into the center by an angular adjustment of the driving drum axes and the return drum axes in accordance with the prior art centering techniques. In the heated-plate region of the press, and in particular in the first one-third of the high-pressure zone, the steel bands are firmly clamped as in a vise. On adjustment of the drum axes over the adjusting path, a somewhat greater elongation results in the intake region than in the clamped region, since in the intake region the steel band is not clamped. The same effect applies to the delivery end. The ratio of the clamped distance in the high pressure region of the increasingly long presses is thus more and more unfavorable in comparison to the intake and delivery sections. In order to avoid a one-sided overstretching of the steel bands on adjusting the course of the steel bands, the press has to be stopped and all the attendant disadvantages endured.